Fix regression when reading `Event::PaymentReceived` in some cases

Fix regression when reading `Event::PaymentReceived` in some cases

For some reason rustc was deciding on a type for the `Option` being
deserialized for us as `_user_payment_id`. This really, really,
absolutely should have been a compile failure - the type (with
methods called on it!) was ambiguous! Instead, rustc seems to have
been defaulting to `Option<()>`, causing us to read zero of the
eight bytes in the `user_payment_id` field, which returns an
`Err(InvalidValue)` error as TLVs must always be read fully.

This should likely be reported to rustc as its definitely a bug,
but I cannot seem to cause the same error on any kinda of
vaguely-minimized version of the same code.

Found by `chanmon_consistency` fuzz target.